1. Field of the Invention
This invention relates to rubber compositions particularly suitable for use in automobile tire treads.
2. Description of the Prior Art
From the safety point of view, spiked tires and tire chains are in common use in the snowy season. The spikes and chain links are brought, while in driving, into severe frictional contact with the road breaking up the road and causing dust hazards after thawing. This poses dangerous as well as environmental pollution problems. Alternatively, improved tires have come to light which are stud-free and have good braking traction even under snowy and icy conditions.
Many attempts have been made to further improve stud-free tires with respect to their braking and traction qualities on snow and ice. It has been proposed to reduce the hardness of tread rubber at low temperature with use of large amounts of softeners and plasticizers as disclosed for instance in Japanese patent laid-open publication Nos. 55-135149, 58-199203 and 60-137945. Excess softener and plasticizer, though making the tread rubber sufficiently resistant to skidding on snow or ice, renders the resulting tire highly abrasive to frictional contact with a road not covered with snow or ice.
Japanese patent laid-open publication Nos. 60-44538, 59-142236 and 53-133248 disclose that high-hardness rubbers having particulate minerals or organic monofilaments can be dispersibly blended with the tread rubber composition. This prior composition when vulcanized, however, is readily susceptible to cracking in the neighbourhood of the dispersed material due to the magnitude of hardness.
A carbon black-free rubber composition has been found effective in improving icy friction. It has also been found that the absence of, or if present, too small a content of carbon black, makes the composition too susceptible to abrasion to warrant commercial application.